The present invention relates to particle beam imaging and measurement equipment in general and in particular to automated in-process scanning electron beam equipment for performing high resolution imaging and measurement of semiconductor wafers having topographic and/or material features.
As integrated circuits are fabricated with features smaller than the wavelength of visible light, electron beam imaging has been established as the technology of choice for process development and quality assurance, as described by M. H. Bennett and G. E. Fuller in an article entitled "In-Process Inspection and Metrology of Semiconductor Wafers with the Use of an Automated Low-Voltage SEM," Microbeam Analysis 1986, pp. 649-652.
When a particle beam impinges on a specimen, there is a release of electrons from the specimen. The electrons are broadly divided into two categories: electrons with energies below 50 electron volts which are referred to as secondary electrons and electrons with energies equal to or greater than 50 electron volts which are referred to as backscattered electrons.
As described by L. Reimer in a book entitled "Image Formation in Low-Voltage Scanning Electron Microscopy", SPIE Optical Engineering Press, 1993, secondary electrons reveal topographic details of the specimen surface whereas, backscattered electrons discriminate between different materials included in the specimen.
U.S. Pat. No. 4,941,980 to Halavee et al discloses an apparatus and method for measuring a topographic feature employing four electron detectors, spaced evenly in a circle around an electron column, which directs a primary electron beam at normal incidence to a specimen. To attract the secondary electrons emitted by the specimen, a positive charge is applied simultaneously to two oppositely disposed electron detectors. As the primary beam is scanned over the surface of the specimen, the signals from the two charged detectors are combined so as to make a cross-sectional profile of topographic features on the specimen. By alternating between the two pairs of oppositely placed detectors, profiles can be obtained along two orthogonal directions. In this prior art, a relatively long working distance of at least several millimeters must be maintained between the specimen and the electron column in order to facilitate collection of secondary electrons by the surrounding electron detectors.
In addition to the measurement methods presented in the aforementioned patent, the signals from the four spatial detectors can be combined using stereoscopic methods described by P. Atkin and K. C. A. Smith in an article entitled "Automatic Stereometry and Special Problems of the SEM", Electron Microscopy and Analysis, Institute of Physics Conference Series Number 68, 1983, pp. 219-222 to provide indirect three dimensional imaging and measurement of the specimen surface.
U.S. Pat. Nos. 4,728,790 and 4,785,176 disclose an electrostatic-magnetic objective lens which provides both high resolution focussing of the primary beam on the specimen and efficient collection of electrons emitted therefrom. A scintillator detector, placed above the objective lens, generates a signal which is proportional to the total electron emission from the specimen. In this prior art, there is no provision for spatial discrimination using multiple detectors, nor is there a provision for separate detection of secondary and backscattered electrons.
U.S. Pat. No. 4,896,036 discloses a detector objective for scanning microscopes consisting of a purely electrostatic objective lens and an annular detector placed above the objective lens. This arrangement has the advantage of preserving the spatial orientation of the detected electrons, by eliminating the Larmor rotation that generally occurs in magnetic objectives. However, electrostatic objectives are known to have large chromatic aberrations, which limit their resolution at low primary beam energies. This is a disadvantage in semiconductor applications, for which low beam energies are needed to avoid specimen charging and damage.
There is therefore a need for particle beam column which provides very high resolution topographic and/or material feature discrimination of a specimen.